Converters which are in the form of so-called resonant converters have a resonant circuit, which can be a series or parallel resonant circuit. When configuring converters, one aim is to keep losses low. Resonant converters which comprise an LLC resonant circuit having two inductances and one capacitance are well-known. Such converters have the advantage that energy-efficient operation with relatively low switching losses is possible.
Resonant LLC converters are well known for use within LED drivers. The converters can be configured or operated as a constant current source or a constant voltage source. A constant current source can be used to drive an LED arrangement directly, thus enabling a single stage driver. Constant voltage sources can be used, for example, for LED modules which have further driver electronics in order to ensure a corresponding power supply to the LEDs with a predetermined current from the output voltage provided by the constant voltage source.
The LLC converter comprises a switching arrangement (called the inverter switch) for controlling the conversion operation, and the switching is controlled using feedback or feedforward control, in order to generate the required output.
Another function implement within a power converter which is supplied with mains (or other AC) power is power factor correction (PFC). The power factor of an AC electrical power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit. A power factor of less than one means that the voltage and current waveforms are not in phase, reducing the instantaneous product of the two waveforms. The real power is the capacity of the circuit for performing work in a particular time. The apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power will be greater than the real power.
If a power supply is operating at a low power factor, a load will draw more current for the same amount of useful power transferred than for a higher power factor.
The power factor can be increased using power factor correction. For linear loads, this may involve the use of a passive network of capacitors or inductors. Non-linear loads typically require active power factor correction to counteract the distortion and raise the power factor. The power factor correction brings the power factor of the AC power circuit closer to 1 by supplying reactive power of opposite sign, adding capacitors or inductors that act to cancel the inductive or capacitive effects of the load.
Active PFC makes use of power electronics to change the waveform of the current drawn by a load to improve the power factor. Active PFC circuits may for example be based on buck, boost or buck-boost switched mode converter topologies. Active power factor correction can be single-stage or multi-stage.
In the case of a switched mode power supply, a PFC boost converter is for example inserted between the bridge rectifier and the mains storage capacitor. The boost converter attempts to maintain a constant DC bus voltage on its output while drawing a current that is always in phase with and at the same frequency as the line voltage. Another switched-mode converter inside the power supply produces the desired output voltage or current from the DC bus.
Due to their very wide input voltage range, many power supplies with active PFC can automatically adjust to operate on AC power for example from about 110 V to 277V.
Power factor correction may be implemented in a dedicated power factor correction circuit (called a pre-regulator), for example placed between the (mains) power supply and the switch mode power converter which then drives the load. This forms a dual stage system, and this is the typical configuration for high power LED applications (for example more than 25 W). The power factor correction may instead be integrated into the switch mode power converter, which then forms a single stage system.
In this case, there is a single resonant tank and switching arrangement, which then implements both power factor correction as well as control of the conversion ratio between the input and output in order to maintain the desired output (current in the case of an LED driver) delivered to the load.
Active power factor correction typically involves providing the input current and voltage waveforms to a controller so that their relative phase angle may be controlled by adjusting the load.
It has been proposed in US 2014/0091718 to use an LLC DC/DC converter, preceded by a rectifier, as a PFC circuit. The LLC resonant converter is frequency controlled, for which an oscillator is used. The control value of the feedback control system is the inverter's switching frequency.
WO 2016/020213 A2 discloses a driver device for driving a load, in particular for driving an LED unit comprising one or more LEDs. The driver device comprises input terminals for connecting the driver device to an electrical power supply for receiving a variable input voltage from the electrical power supply, a converter unit for converting the input voltage to an output voltage including a resonant converter and a switch unit, wherein the switch unit is adapted to provide a chopped voltage of the input voltage as a drive voltage to the resonant converter, and a control unit connected to the switch unit for controlling a pulse frequency of the chopped voltage, wherein the control unit is adapted to control an input current drawn from the electrical power supply by controlling the pulse frequency of the chopped voltage on the basis of a measured value of the variable input voltage.
Self-oscillating resonant converter circuit are also known which make use of internal components to form a resonant tank, and signal values (e.g. voltage levels which arise in the circuit) are used to implement switching operations. For example, U.S. Pat. No. 8,729,830 discloses the control of a resonant DC/DC converter in a self-oscillating manner, by using threshold detection of states in the resonant tank in order to determine the inverter switching times rather than employing an oscillator and frequency control.
LLC DC/DC converters are either operated at a DC supply voltage (e.g. 48V in telecommunications or data center applications), or they are used as the second stage of a mains power supply or two stage LED driver, in which the front end stage (the power factor correction pre-regulator) provides the power factor correction and also generates a stabilized bus voltage that forms the DC input voltage for the LLC.
Standard power factor pre-regulators with medium and low output power are realized by buck, boost, buck-boost or flyback converters operating in boundary conduction mode (or ‘critical conduction mode’). A flyback converter is typically used if the converter has to be mains isolated. This operation mode is widely used in all kinds of commercial products which have to meet mains harmonic regulations. Typically, for the control of these converters two concepts are used:
(i) Peak current mode control in combination with a multiplier of the mains input voltage for providing a set-point of the mains input current. Here, the input current is closed-loop controlled and has to be measured by means of a current sensor.
(ii) Constant on-time operation of the inverter switch. This approach does not control the mains input current in a closed loop and the control system is very simple to realize. This approach neither needs a multiplier to calculate a set point nor a current sensor. This approach is based on a constant on-time, and the mains input current is roughly proportional to the mains voltage (under certain design and operation conditions).
This invention relates in some aspects to DC/DC resonant converter architectures and in other aspects to resonant LLC converters employed as an AC/DC converter implementing power factor correction (PFC).
An example of a resonant AC/DC converter is shown in FIG. 1. The LLC resonant circuit forms a PFC stage and may thus be used as a PFC pre-regulator by having a controlled output voltage. It could also be used as a single stage LED driver by having a controlled output current.
The circuit comprises a mains input 10 which is followed by a rectifier bridge 12 having a smoothing capacitor 14 at the output.
The converter comprises a primary-side circuit 16 and a secondary side 18. There is electrical isolation between the primary-side circuit 16 and the secondary side 18. A transformer comprising a primary coil 20 and a secondary coil 22 is provided for the isolation. The transformer has a magnetizing inductance 20 which also acts as one of the inductances of a series LLC resonant circuit. The LLC resonant circuit has a second inductance 24, and a capacitance (formed as two capacitors 26 and 27 in this example).
In an LLC circuit, the inductances and capacitor may be in any series order. The inductor may comprise discrete components or it may be implemented as leakage inductances of the transformer.
The primary-side circuit 16 comprises a half-bridge having a first power switch 28 and a second power switch 30. The first switch and the second switch can be identical, and the half-bridge can be in the form of a symmetrical half-bridge. These switches can be in the form of field-effect transistors. The resonant LLC circuit is connected to a node between the two switches.
Each switch has its timing of operation controlled by its gate voltage (shown schematically as a voltage source). Feedback is used to determine the timing of the control of the switches 28, 30.
During operation of the converter, a controller controls the switches, at a particular frequency and in complementary manner.
The circuit shown in FIG. 1 is thus an AC/DC PFC single stage converter, comprising an AC input 10, a rectifier 12, a half bridge inverter comprising a high side switch (the first power switch 28) and a low side switch (the second power switch 30), wherein an output is defined from a node between the switches. The self-oscillating LLC circuit 20,24,26,27 is coupled to the output. A control circuit (not shown) is used for generating a gate drive signal for controlling the switching of the high side and low side switches. A high gate drive signal turns on one switch and turns off the other switch and a low gate drive signal turns off the one switch and turns on the other switch.
In one known approach, the primary-side circuit 16 detects a variable which indicates an average value over time of a current flowing in the circuit, for example through the first or second switch. Information about the load is derived on the basis of the measured current in the primary-side circuit. The measured current may have a direct relationship with the load.
The secondary side 18 has a rectifier which is connected downstream of the secondary coil 22 and which can be formed, for example, by a first diode arrangement 32 (of diodes 32a and 32b) and a second diode arrangement 34 (of diodes 34a and 34b). FIG. 1 shows a full-bridge rectifier and a single secondary coil which couples at its ends to the rectifier circuit. Instead, a center of the secondary coil 22 may be coupled to an output of the secondary-side circuit. The ends of the secondary coil 22 may then be coupled to the output via a half bridge rectifier with only two diodes. The storage capacitor 36 is connected between the outputs of the rectifier. The LED load or other output stage is represented by a resistor. It comprises an LED or a plurality of LEDs.
A control scheme is required to drive the switches 28, 30 into their on- and off-states such that the output voltage or current is regulated to a certain desired value or range of values and for a PFC circuit also to implement power factor correction.
In order to exploit best the powertrain and to achieve the maximum efficiency, it is desired to operate the converter symmetrically (at least at full load) and to load the transformer and the rectifier in the secondary side equally. In the case of a transformer with center-tapped output windings that are symmetric in terms of turn-ratios and leakages, secondary side symmetry can be assured if the duty cycle of the half-bridge (i.e., its switch node) is kept at 50%.
There are basically four transitions that the control scheme must handle:    1. Turn-on of the high-side MOSFET 28;    2. Turn-on of the low-side MOSFET 30;    3. Turn-off of the high-side MOSFET 28;    4. Turn-off of the low-side MOSFET 30.
There are several known schemes that may be used in order to achieve this.    A. Von-Voff is a control scheme where transition number 4 is initiated when some state variable crosses a certain threshold voltage (Von). Following this, the control waits for a certain time (i.e., the dead-time) before starting transition 1. This dead-time ensures that cross-conduction, or shoot-through, does not occur. The half-bridge is now in the on-state. Eventually, either the same or a different state variable will cross a second threshold (Voff), and transition number 3 will be initiated. As with the transition to the half-bridge on-state, there will then be a dead-time before transition number 2 is initiated. The half-bridge is now in the off-state, and then the procedure continues from the beginning once more. The actual values of the two thresholds are determined by an outer control loop in order to yield the correct output. This is a Von-Voff scheme in that voltage threshold controls the switching on and off.    B. Von-Ton is a control scheme where transition number 4 is initiated when some state variable crosses a certain threshold voltage (Von). As in case A, the dead-time is allowed to pass before starting transition number 1. Transition number 3 is initiated based on a certain time interval elapsing. This may be a fixed interval, or a controlled interval. After the dead-time has then elapsed, transition number 2 is initiated, and then the procedure continues from the beginning once more. The actual value of the voltage threshold is determined by an outer control loop in order to yield the correct output, and the time threshold may be fixed or controlled dynamically. This is a Von-Ton scheme in that a voltage threshold controls the turning on (after a dead time) and the time duration of the on period of the half bridge is then controlled.    C. Voff-Toff is similar to case B, except that the voltage and time thresholds define the off and on transitions of the half-bridge, respectively. Transition number 3 is initiated when some state variable crosses a certain threshold voltage (Voff). The dead-time is allowed to pass before starting transition number 2. Transition number 4 is initiated based on a certain time interval elapsing. After the dead-time has then elapsed, transition number 1 is initiated, and then the procedure continues from the beginning once more. As in case B, the actual value of the voltage threshold is determined by an outer control loop in order to yield the correct output, and the time threshold may be fixed or controlled dynamically. This is a Voff-Toff scheme in that a voltage threshold controls the turning off and the time duration of the off period of the half bridge is controlled (i.e. between turning off the high-side MOSFET and turning it on again after the time duration and dead-time).
In cases B and C, it is most often desirable to control the on (off) time such that it matches the off (on) time, i.e., it is usually beneficial to operate with a 50% duty cycle as mentioned above. In other cases, it is beneficial to operate with a defined duty cycle that is different from 50% in order to enlarge the output voltage or current window that the converter is capable of handling.
For threshold-based resonant converters (such as a self-oscillating LLC converters), there is no oscillator present in the circuit. Threshold-based switching has a particular advantage with regards to the linearity of the transfer function when using the converter to cover a wide range of input and output operating conditions, such as in an LLC PFC for example, and frequency control is not feasible in such cases due to extreme variations in the gain that cannot easily be handled.
This invention relates to improvements to the design and control of resonant LLC converter circuits and in particular for use as PFC pre-regulator circuits, although some aspects also apply to DC/DC converters, and some aspects also apply to single stage PFC drivers. Some aspects are also not limited to LLC circuits.